A fuel vapor retention system is disclosed, for example, in van Basshuysen & Schäfer, “Handbuch Verbrennungsmotor”, 2nd edition, Vieweg Verlag, 2002, pages 604 to 607. Such a fuel vapor retention system is provided within a motor vehicle, for example, in order to absorb and store fuel vapor that forms in a fuel tank due to evaporation, with the result that the fuel vapor cannot escape into the environment. A fuel vapor retention filter is provided within the fuel vapor retention system as a store for the fuel vapor, said filter using, for example, activated carbon as the storage medium. The fuel vapor retention filter only displays a limited storage capacity for fuel vapor. In order to be able to use the fuel vapor retention filter over a long period, said filter must be regenerated. During regeneration, the combustion engine draws. in the fuel vapor stored in the fuel vapor retention filter. The fuel vapor is thus fed to the combustion in the combustion engine and the absorption capacity of the fuel vapor retention filter for fuel vapor is thus restored. A valve of the fuel vapor retention system is arranged between the fuel vapor retention filter and a suction pipe of the combustion engine for the purposes of dosing the fuel vapor quantity that the combustion engine draws in from the fuel vapor retention filter.
DE 10 2004 022 999 B3 discloses a method for determining a control characteristic for a valve of a fuel vapor retention system of a combustion engine. The control characteristic represents a current relationship between a pulse-width-modulated control signal being used for activating the valve and a valve position being set. A currently valid minimum pulse width of the control signal, which is currently required for opening the valve, is determined by increasing the pulse width in steps up to the detection of a deviation of the instantaneous behavior of the engine with respect to a steady-state behavior of the combustion engine. Said increasing of the pulse width in steps starts at a predefined value of the pulse width that is greater than zero and smaller than a value corresponding to a minimum pulse width determined at an earlier time point.